The invention relates to an eddy current sensor with an exploring coil wound on a coil form, with two terminals, a source of ac voltage, an electrically conductive measuring probe, and an evaluation circuit, wherein the measuring probe is displaceable relative to the exploring coil, and the evaluation circuit generates an evaluation signal as a function of the position of the measuring probe.
An eddy current sensor of the described type is disclosed in U.S. Pat. No. 5,629,619. For evaluating the position of the measuring probe, a plurality of voltage taps are provided. The different potentials applied to the voltage taps are supplied to an evaluation circuit. The measuring probe arranged between two voltage taps each influences the partial impedance of the corresponding coil section and, thus, the potential on the voltage taps. With the aid of the evaluation circuit, it is therefore possible to determine the position of the measuring object with respect to the voltage taps. However, this arrangement involves the disadvantage that the ratio of the measuring range to the length of the sensor is dependent on the number of voltage taps. To obtain a measuring range as large as possible with respect to finding the position of the measuring probe, it is necessary to have a large number of voltage taps of the exploring coil, which increases the complexity of the measuring sensor. At the same time, it is also necessary to adapt the length to the measuring probe, which is dependent on the spacing of the voltage taps.
It is therefore the object of the present invention to improve an eddy current sensor of the initially described kind such that a simple construction of the eddy current sensor permits realizing a greatest possible ratio O)f the measuring range to the length of the sensor with respect to measuring the position of the measuring probe.
A further object of the invention is that the sensor exhibits a satisfactory temperature stability.
The above and other objects and advantages of the present invention are achieved by the provision of an eddy current sensor which comprises an electrode with a tap for enabling electrical contact. The electrode forms together with the windings of the exploring coil and an intermediate electrically insulating layer a component with distributed electromagnetic parameters, i.e. complex impedances, whose output signals are used to determine the position of the measuring probe.
According to the invention, the eddy current sensor has therefore both inductive and resistive as well as capacitive components. While the measuring probe influences only that part of the exploring coil, which it covers, the impedance of the exploring coil as a whole is independent of the position of the measuring probe. In this connection, the extension of the measuring probe parallel to the coil axis may be very small, theoretically infinitely small.
It is therefore possible to make available a high-resolution eddy current sensor with a ratio as high as 90% between the measuring range and the length of the sensor, with only two terminals on the exploring coil and a further tap of the electrode. The two terminals are connected in particular indirectly, i.e., by means of wiring, or directly to the terminals of a source of ac voltage.
The simple construction of the eddy current sensor makes it possible to produce it in a cost-favorable manner. It has only three terminals for contacting respectively the voltage supply and the evaluation circuit. Consequently, it is possible to make the eddy current sensor even more compact, since the width of the measuring probe may be very small, and the exploring coil may also have a smaller length in the axial direction. Moreover, the dimensions of the measuring probe influence the measuring result to a lesser extent than in the solution with the voltage taps. As a result, the eddy current sensor of the present invention is also very suitable for miniaturization.
It is preferred to construct the intermediate layer as an electrode, as a conductor, or a printed circuit board, and to arrange it parallel to the axis of the exploring coil. The parallel arrangement generates an impedance with inductive, capacitive, and ohmic resistance components, with the capacitive component being formed between the intermediate layer and the windings of the exploring coil. In connection with an axis parallel displacement of the measuring probe, the component with distributed electromagnetic parameters is also influenced in ranges arranged parallel to the axis of the exploring coil, which leads to output signals that can be used for a simple evaluation by an evaluation circuit.
In a further development of the invention, the entire coil form is made an electrode and consists of a material with little electric resistance, in particular a ferromagnetic material. This makes it possible to produce on the one hand the coil form in a simple shape from uniform material, and, advantageously, the electrode has a particularly large surface. The coil form may be designed and constructed as a tubular component.
An intermediate layer is arranged between the windings of the exploring coil and the coil form. This intermediate layer may be applied to the surface of the coil form. As a result of the material properties and the geometry of the intermediate layer (electric conductivity xe2x80x9cxcex4xe2x80x9d, relative permittivity xe2x80x9cxcex5xe2x80x9d, thickness), the transition resistance is variable between the windings of the exploring coil and the electrode.
In this connection, the exploring coil may be wound from insulated or bare wires. If bare wires are used, the windings will have to be wound in a certain spaced relationship between one another.
In another further development of the invention, the insulating layer of the winding wire is used as intermediate layer between the coil form and the electrode. In this instance, the capacitive component of the eddy current sensor dominates, since the ohmic resistance of the intermediate layer is very high.
However, if the ohmic component R is smaller than the capacitive component C of the intermediate layer and, thus, substantially greater than the impedance of the exploring coil {dot over (Z)}I per unit of length, the output signal will depend substantially on the ohmic component R.
In another advantageous further development of the invention, the coil form is an insulator, and a casing surrounding the exploring coil forms the electrode. In this arrangement, the casing may be made from a conductive, nonferromagnetic material, in particular a stainless steel. In particular in connection with a measuring probe extending inside the coil form, it is thus possible to produce a compact and encapsulated eddy current sensor. The intermediate layer may comprise a complex transition resistance between the windings of the exploring coil and the electrode with a negative or positive temperature coefficient. This permits compensating the temperature influence on the output signal of the eddy current sensor.
Preferably, the electric resistance value of the electrode per unit of length is much greater than the resistance value of the measuring probe per unit of length. This permits influencing the eddy current effects in the electrode and measuring probe in a purposeful manner.
The measuring probe is an electrically conductive ring surrounding the exploring coil and adapted for noncontacting displacement on the outer surface of the exploring coil along the axis thereof. However, the measuring probe may also be arranged for displacement inside the coil form in a channel extending parallel to the coil axis. The measuring probe thus extends protected inside the exploring coil, and it may be constructed very small. In this connection, the measuring probe may also be part of a measuring object or be the measuring object itself. In the last-mentioned case, it is possible to use particles arranged inside a tubular coil form to form a measuring probe.
In a particularly preferred embodiment, the electrode, intermediate layer, and coil form are each aligned with their longitudinal axes parallel to the axis of the exploring coil, and/or the deflection of the measuring probe occurs parallel to the axis of the exploring coil. This forms a simple, linear arrangement, which generates easy-to-evaluate output signals for an evaluation circuit.
In another preferred further development of the invention, the coil form forms a closed ring, if need be, with the electrode, and the measuring probe is supported for displacement along the ring. In this connection, the measuring probe may again form a ring surrounding the coil form and the exploring coil, or be supported in a channel inside the coil form. This embodiment is especially suitable for detecting angular changes, which are performed by a measuring object that is connected to the measuring probe.
In a preferred further development of the invention, both terminals of the exploring coil connect to a source of ac voltage, and the evaluation circuit comprises an operation amplifier, whose input connects to the tap of the electrode. In accordance with the invention, the inverting input of the operation amplifier may be connected to the tap of the electrode. Due to the change in the output signal on the eddy current sensor, i.e., the voltage change, the evaluation circuit detects the position or position change of the measuring probe relative to the exploring coil.
In another further development of the invention, ac voltage is applied between the tap of the electrode and the terminal of the exploring coil, and the second terminal of the exploring coil connects to an input of an operation amplifier of the evaluation circuit. Preferably, the terminal of the exploring coil connects to the inverting input of the operation amplifier. Likewise, this evaluation circuit is wired by resistors and capacitors in such a manner that it makes it possible to compare or add output voltages.
In another further development of the invention, the exploring coil comprises a further terminal, in particular in the form of a center tap of the exploring coil. The center tap connects via a low-pass filter to an inverting input of an operation amplifier. The output signal may be used for compensating the temperature gradient influence of the impedance of the eddy current sensor.
Finally, the tap of the electrode may also connect to ground. In this instance, the two terminals of the exploring coil connect, for example, to a source of ac voltage and to the input of an operation amplifier.
All components of the sensor may be miniaturized on a chip, for example, as a magnetoresistive or photoresistive structure.